Sometimes though, I want to control who fills out my form, or who even sees the questions I am asking. That's when I add a password. Okay, technically it is not really a password. I just make the first question on the form a text box that requires a specific word before the user can go on to the next page. (Also, don't call it a password or Google could think you're phishing and shutdown your form.)
My current use of this is for quizzes in my classroom. I used to write a short link on the board when I wanted students to take the quiz. By withholding the link I could be sure students would not take the quiz before they came to class, or look at the questions ahead of time. Now, I post the link to the form on my class blog, then I give the students the "entry code" when I am ready for them to start.
I'm not paranoid about quiz security. I'm not going to change the entry code every period. I don't worry about kids in period one sharing the code word with kids in period 6. These aren't high stakes assessments. I also enjoy making the code word something related to our unit of study, or a spelling word students will need to look up before they can take the quiz.
I also use entry codes on other kinds of class work to be sure students listen to directions before they rush to get started on filling out a form.
Having a code word on the form also prevents people not in my classes from filling out a form meant only for my students. Our class blog gets lots of visitors and I'd rather not have to scrub the data if other people are filling out my forms. (People still fill out quizzes about Gatsby that I haven't used in three years because they find them on my old class blog.)
For exact directions on how this works check out this tutorial video I made for you. And remember, don't call it a password. Entry code works much better.